1. Field of the Invention
The present invention is directed to a mobile nuclear magnetic resonance apparatus having a shielding arrangement with at least one primary shielding coil.
2. Description of the Prior Art
Mobile nuclear magnetic resonance apparatuses of the above type are well-known and are mounted on, for example, a truck trailer for a mobile installation. The nuclear magnetic resonance apparatus is permanently installed in the cargo space of the truck. It must be assured during the transport of a mobile nuclear magnetic resonance apparatus that the stray field outside the vehicle is low (even close to the exterior of the vehicle). The stray field is that part of a magnetic field outside the patient tube that does not contribute to the imaging. The allowable limit value for the stray field currently amounts to 5 mT, so that heart pacemakers are not influenced.
In known mobile nuclear magnetic resonance apparatuses, the stray field is shielded by an active shielding of the magnet with primary shielding coils in combination with a passive flux return through non-removable iron plates. The primary shielding coils are arranged in the magnet and are dimensioned so that they do not influence the homogeneity volume. United Kingdom Patent 2 162 641 discloses primary shielding coils for reducing the basic magnetic field outside the examination volume in a mobile nuclear magnetic resonance apparatus. In order to assure the strong shielding of the magnet toward the outside required for the transport, a passive flux return must be undertaken in such a known mobile nuclear magnetic resonance apparatus. These shielding measures have the disadvantage that the field in the examination volume is disturbed even by the slightest movements of the iron plates, for example due vibrations or fluctuations in air pressure in the cargo space, and thus in the patient tube.
The quality of the exposures, which require a field homogeneity precision in the ppm range, is reduced due to these field instabilities.